This invention relates generally to metal transfer, and, more particularly, to a segmented trough for conveying molten metal.
Industrial plants such as mills, foundries, or casting houses which utilize molten metal must often transfer large amounts of molten metal from one location to another. As an example, molten metal may be supplied to a central casting facility from several different melting or refining furnaces, which, because of space limitations, may be placed some distance from the casting facility. Molten metal must therefore be transferred from the melting furnaces to the casting unit.
One approach to moving molten metal is the use of ladles or transfer cars into which all or part of the molten metal in the furnace is loaded as a batch, and then moved to the place where the molten metal is needed. Such batch transfer arrangements are ordinarily used where the molten metal is to be transferred a long distance such as many hundreds of feet. The batch transfer methods are not convenient where the metal is to be transferred a shorter distance, where the metal is needed in a congested location, or where the metal is to be supplied at a low flow rate over an extended period of time. In these latter circumstances, metal may be conveniently transferred relatively short distances using a transfer trough.
As an example of the desired use of transfer troughs, in an aluminum continuous casting operation aluminum alloys may be melted in several melting furnaces, and the metal conducted to a holding furnace by a transfer trough for mixing and temperature equilibration. From the holding furnace, the metal is further transferred to a continuous casting unit such as a Properzi bar casting wheel. In such applications, the rate of metal flow is very low, as on the order of 300 pounds per minute. For comparison, in other operations the metal flow rate may be thousands of pounds per minute. The low metal flow rates require that little heat be lost per foot of trough length. Also, the Properzi wheel may be continuously operated for 30 or more hours, which requires that the trough have excellent long term stability.
A molten metal transfer trough typically includes a channel for conducting the molten metal, formed from a refractory material which is not wetted by the molten metal and withstands the high heat of the metal. The refractory has a low heat transfer coefficient to minimize the heat loss by conduction as the molten metal passes along the trough. Because such refractories are generally rather brittle, the refractory must be supported within a steel bottom shell or frame which also has provision for attaching adjacent portions of the trough together.
The trough is sometimes provided with a cover to reduce the heat loss by radiation from the molten metal and also to minimize oxidation of the molten metal as it passes along the trough. A cover for a molten metal transfer trough may be as simple as a block of refractory laid on top of the trough and covering the channel, or may be more complex such as a refractory layer mounted in and supported by a metal cover shell, which cover shell may be hinged to the bottom shell for convenience in opening the top. Another desirable feature is to provide the cover with a heater to preheat the trough before metal is passed along the trough, thereby reducing or eliminating any freezing of the first molten metal to pass down a cold trough.
While molten metal troughs of the type described are often satisfactory in conveying molten metal from one place to another, they suffer from serious problems arising from the differences in thermal expansion coefficients of materials as they are heated and cooled. The problems are particularly troublesome for troughs used for extended periods of time. Most materials expand in length and volume when heated, and contract upon cooling. Different materials expand and contract at different rates. The rates may be measured and are known for most common materials. Refractory materials such as those used in the trough channel have relatively small coefficients of thermal expansion, while the steel used in the shells has a relatively large coefficient of thermal expansion. As molten metal passes along the trough, the different heating rates, temperature rises, and coefficients of thermal expansion of the various components of the trough result in differential strains and stresses within the trough. When the differential strains and stresses are sufficiently large, they cause the formation of cracks in the brittle refractory material and the leakage of molten metal to the steel shell. Ultimately, the steel shell may melt so that the entire trough fails.
The coefficients of thermal expansion of materials are fixed. Although a great deal of research has been devoted towards improved refractory materials having increased resistance to failure under thermal cycling conditions, no material has been discovered having the necessary properties and also having a sufficiently low cost to justify its use in mill applications. Thus, while improvements in the materials themselves are possible, no generally satisfactory material system has been found to allow the use of conventional troughs in many operations.
In another approach, water cooling tubes or jackets have been applied to the outside of the steel shell of the trough to keep the steel cool, thereby reducing the possibility of catastrophic failure. This trough construction is unsatisfactory, because water is brought near to the molten metal. If in an accident the molten metal should contact the water to form steam, there may be a violent explosion of metal. Also, using water cooling on the outside of the trough accelerates total heat loss, so that the trough cannot be used to convey very low flow rates of metal, over long distances.
No one seems to have previously recognized the nature of the differential expansion problem, and particularly its significance for troughs to be used in low metal flow, long time operations. No satisfactory metal transfer trough has therefore been devised for use in such operations.
Accordingly, there has been a need for an improved molten metal transfer trough having the desirable qualities of previously existing troughs, but also having reduced susceptibility to material damage or leakage of molten metal resulting from the effects of differential thermal expansion of the components. Such a trough should have a very low rate of heat loss per foot of length and be stable for use in long continuous operation. The present invention fulfills this need, and further provides related advantages.